1. Field of the Invention
This invention relates to a method and apparatus for the production of metal or alloy powder (hereinafter referred to collectively as "metal powder") by atomizing molten metal.
2. Description of the Prior Art
The metal powder which is used as the raw material for powder metallurgical articles such as powder-sintered articles and powder-forged articles generally is required to possess the following attributes:
(1) The metal powder should have a low oxygen content and sparingly yield to surface oxygen.
(2) The metal powder should have a proper particle size distribution and contain finely divided particles in a proportion above a certain level.
(3) The individual particles of the metal powder should have irregular shapes.
As ways for producing metal powder, many methods ranging from mechanical methods to chemical methods have been introduced to the art. Among other methods, the so-called atomizing method, i.e., a method by which a stream of molten metal is reduced into finely divided particles by the force of an injected current of water or gas has found widespread acceptance owing to its advantage in terms of the properties of the metal powder produced, and to the efficiency and cost of the production technique. However, considering the aforementioned conditions the metal powder is required to fulfill, the water atomizing method and the gas atomizing method have various problems of their own, as indicated below.
The water atomizing method effects the reduction of molten metal into finely divided particles by a jet of water. In the course of this atomization, the divided particles of metal undergo surface oxidation by the ambient air and the water from the jet. When this method is carried out on molten iron in an air atmosphere, for example, oxygen in an amount of about 3 to 5% based on the weight of iron being atomized reacts with the iron to produce iron oxide. It is known that oxygen in an amount of about 0.2 to 0.5% inevitably reacts with the iron even when the atomization is carried out in an inner gas atmosphere and an ingenius device is used for the injection of the atomizing agent. To permit production of shaped articles of high mechanical properties, the metal powder obtained by the water atomizing method must be given an additional reducing treatment before it is put to use.
This reduction necessitates provision of such a reducing substance such as H.sub.2 or CO and requires the ambient temperature to be elevated above 1000.degree. C. The equipment for reduction is composed of heating facilities, facilities for powder transfer, facilities for preparation of a reducing gas, and cooling facilities. The cost for the provision of the reducing gas and for the elevation of ambient temperature accounts for a high proportion of 20 to 30% of the total cost of equipment. Use of these extra facilities constitutes one of the factors for increasing the price of the metal powder itself and the price of shaped articles obtained from the metal powder as the raw material.
In the case of the atomizing method using a gaseous agent, the possible oxidation during the production of metal powder may be precluded by using a non-oxidizing gas such as inert gas, neutral gas, or reducing gas in high purity. The aforementioned reducing treatment, therefore, can be omitted. Nevertheless, this method entails the following problem.
For the convenience of cold forming, the metal powder is required to meet the condition that the individual particles of the powder should possess surface shapes irregular to a certain degree as mentioned above. According to the gas atomizing method, however, since the gas has low specific heat and density, no sufficient cooling speed can be obtained. Consequently, the atomized particles of molten metal, while being cooled, are caused by surface tension to convert themselves into smooth spheres. The metal powder thus made up of spheres of smooth surface has a disadvantage that it exhibits poor cold formability and fails to give compressed powder of sufficient strength and produce sintered articles of ample strength.
Recently, there have been several proposals for a method of using oil as an atomizing medium, the proposals of which being, for example, U.S. Pat. No. 4,124,377 to Larson, Rumanian Pat. No. 51,997, and Int. Powder Metall Conf. (U.S.A.) 301 to 311 ('74).
The oil atomizing method, which uses a varying kind of oil instead of gas or water, enjoys a high cooling efficiency and removes the possibility that the powder particles will be exposed to oxidation. This method, nevertheless, is susceptible to the following problems and is not yet practically used.
(1) When the molten metal is atomized with an oil, there is required an additional treatment for separating and recovering the oil adhering to the divided particles of the metal.
(2) The oil, on contact with the molten metal at a high temperature, is decomposed to produce a carburizing atmosphere, with the result that the produced metal powder acquires an increased carbon content. As a result, the product is made so hard as to be considerably difficult to exhibit cold formability.